1. Field of the Invention
The present invention relates to an opto-magnetic recording medium used for an opto-magnetic memory, a magnetic recording device or a display device and from which information is read-out by utilizing a magneto-optical effect such as magnetic Kerr effect or Faraday effect.
2. Description of the Prior Art
There have been developments relating to optical memories in which information recorded on various recording media is optically read, and such memories are attracting much attention for high-density large-capacity information storage as a result of recent progress in the laser and optical elements. Among such recording mediums there is the so called opto-magnetic recording medium in which magnetically recorded information is read by utilizing a magneto-optical effect. Such magneto-optical recording medium is provided, as a recording layer, with a magnetic layer that is easily magnetized in a direction perpendicular to the layer surface. Information recording is, for example, achieved by magnetizing said recording layer in one direction and partially heating said layer with a laser beam spot to invert the direction of magnetization in said heated spot. The information can be read optically from such information bearing medium by means of the magnetic Kerr effect or Faraday effect. Also the recorded information can be erased by partial or total heating of the medium in an externally applied magnetic field. The magnetic recording layer was originally composed of a polycrystalline thin layer such as MnBi, but recently investigated are amorphous alloy thin layers of transition metals (Fe, Co, Ni, etc.) and rare earth metals (Tb, Dy, Gd, Ho, etc.) represented by a ternary alloy GdTbFe, in order to achieve magnetically uniform film formation, a suitable information recording temperature and a large magneto-optical effect.
On the other hand, such opto-magnetic recording media are associated with a drawback of a low level of the reproduced signal. Particularly in signal reproduction through the Kerr Effect, it has been difficult to increase the signal-to-noise (S/N) ratio because of a small Kerr rotation angle. For this reason, there have been made various proposals for increasing said Kerr rotation angle through the improvement of the magnetic material constituting the recording layer, or formation of a dielectric layer of SiO or SiO.sub.2 on the recording medium to utilize the multiple reflection on the recording layer.
Also, as disclosed in the Japanese Laid-open Patent Applications Nos. 6541/1983 and 6542/1983, it is proposed to utilize a sufficiently thin magnetic recording layer with a reflective metal layer on the back side, thereby obtaining a large reproduced signal through the combination of Kerr effect and Faraday effect. FIG. 1 of the accompanying drawings shows an example of such recording medium, comprising a translucent substrate 11, a magnetic recording layer 12, a reflective metal layer 13 and a protective layer 14. It is also reported (J. Appl. Phys., Vol. 53, No. 6, p. 4485-4494) that an even higher S/N ratio can be obtained, in a recording medium having a transparent dielectric layer between the magnetic recording layer and the reflective metal layer as shown in FIG. 2 of the accompanying drawings by the suitable selection of materials and thicknesses of said three layers. In FIG. 2 there are shown a translucent substrate 21, a magnetic recording layer 22, a transparent dielectric layer 23, a reflective metal layer 24 and a protective layer 25. Said transparent dielectric layer 23 functions to generate a phase difference between the light reflected at the interface between the amorphous magnetic layer 22 and the translucent opto-magnetic substrate 21, and the light reflected by the reflective metal layer 24 and reaching said interface, thereby reducing the reflectivity of the media and apparently increasing the angle of rotation of the plane of polarization by the magnetooptical effect.
However, such conventional opto-magnetic recording media have been easily oxidized, since the reflective layer is composed of a thin metal layer of a thickness of 100-600 .ANG.. The oxidation of the reflective layer leads to lowered recording sensitivity and an increase in errors and signal deterioration in the signal readout, and eventually gives rise to the potential for oxidation of the recording layer. Also it is difficult to achieve a high S/N ratio by means of the above-explained transparent dielectric layer, since a uniform layer thickness cannot be reproduced precisely.
On the other hand, an opto-magnetic recording medium which eliminates the above-noted disadvantages by forming the reflective layer of a mixture of a substance having small absorption for a light of a predetermined wavelength and a substance having great reflection for such light has been proposed by the inventors.